Backlighting device for the display screen of a television or mobile phone

ABSTRACT

Backlighting device for a screen for a television, mobile phone or the like, wherein the backlighting device comprises:
         a first light source adapted to emit light having a peak wavelength between 600 and 630 nm;   a second light source adapted to emit light having a peak wavelength between 510 and 530 nm;   a third light source adapted to emit light having a peak wavelength between 440 and 460 nm;   wherein the light emitted by one of the light sources has a bandwidth of less than 15 nm, preferably less than 10 nm, more preferably less than 5 nm.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to backlighting device for a screen for atelevision, mobile phone or the like. The invention also relates to ascreen comprising such a backlighting device.

2. Description of the Related Art

Presently, screens and displays for televisions or the like are oftenmade of LCDs (liquid crystal displays) which are backlit using LEDs(light-emitting diodes). The LEDs produce light with a certain spectraldistribution which is guided through a plurality of which may be TFT(thin-film transistor) channels with a well-defined bandwidth selectionprofile, in order to provide coloured subpixels in red, green or blue.The screens are typically compliant with international standards such asBT 2020 or NTSC. Typically, an average of about 6% of the light producedbehind the TFT channels is actually emitted from the front of thescreen. This limits the luminous efficiency of the screen and forms awaste of energy.

At the same time, the perceived resolution of the screen is limitedbecause of an overlap in the emission wavelengths that pass through thedifferently coloured TFT channels. This may cause the human eye toperceive multiple adjacent subpixels, which are intended to havedifferent colours, as single subpixels. This may reduce the perceivedresolution and deteriorates the visual acuity.

In light of the above, it would be desirable to provide solutions whichat least partially overcome some of the inconveniences of the prior art.

SUMMARY OF THE INVENTION

According to the invention there is provided a backlighting device for ascreen for a television, mobile phone or the like, wherein thebacklighting device comprises:

-   -   a first light source adapted to emit light having a peak        wavelength between 600 and 630 nm;    -   a second light source adapted to emit light having a peak        wavelength between 510 and 530 nm;    -   a third light source adapted to emit light having a peak        wavelength between 440 and 460 nm;    -   wherein the light emitted by each or one of the light sources,        preferably at least the second light source, has a bandwidth of        less than 15 nm, preferably less than 10 nm, more preferably        less than 5 nm. For the present purpose, bandwidth is the        spectral bandwidth in wavelength and defined as the full-width        at half maximum (FWHM), as is common in the art and references        to bandwidth are to be understood as being the FWHM bandwidth.        The bandwidths referred to are measured at room temperature.

In the following, the spectral peak of the first light source may bereferred to as the red peak, the spectral peak of the second lightsource may be referred to as the cyan peak, and the spectral peak of thethird light source may be referred to as the blue peak.

The invention also relates to a screen for a television, mobile phone orthe like comprising such a backlighting device.

The spectral power distribution emitted from the backlighting device(also referred to as lighting device) enhances visual acuity and coloursensation for viewers of the screen comprising the backlighting device,since the emission peaks of the different light sources are bettersegregated than in screens of the prior art. The specific spectral powerdistribution also increases perceived resolution since the individualpixels can be better distinguished by a human eye. Due to the spectraldistribution which is suited to the spectrally shifting sensitivity of ahuman eye, the perceived brightness is also increased. This means thatlower lighting powers are sufficient to achieve the same perceivedbrightness, which leads to energy savings.

The concept of improved visual acuity using a modified spectral powerdistribution has been discussed in WO 2015/034350 by the same applicant,which is incorporated herein by reference in its entirety.

The spectral power distribution of the lighting device is different fromspectra defined in common standards for screens and displays such as BT2020 (rec. 2020) or NTSC, which require filters centred around 460, 535and 625 nm. In fact, the present standards do not correspond with thespectral power distribution that is most suited to the human eye. As aresult, not only the lighting device within the screen but also otheroptical equipment in the screen is usually adapted to the standards andthus partially incompatible with the lighting device as described above.

The spectral power distribution emitted by the lighting device asdescribed herein is adapted to the human eye. In particular, the eye'ssensitivity to colour differences is low at around a wavelength of 555nm. Emitted light around this wavelength is therefore difficult todistinguish from light with a slightly different wavelength, which leadsto a reduced colour contrast and therefore lower visual acuity. In otherwords, light emitted around this wavelength does not effectivelycontribute to the perceived acuity. The probability that a pixelemitting light around this wavelength can be distinguished from thesurrounding pixels is relatively small since it requires a largespectral difference in order to be distinguished. Therefore, a largepeak around 555 nm is detrimental for the perceived resolution of thescreen. In the invention, light around this wavelength is thereforeomitted as much as reasonably possible and instead a cyan peak between510 and 530 nm is introduced by the second light source and theamplitude of the red peak is increased. This has been explained in termsof Cone Sensitivity Difference (CSD) in a scientific article in OSAOptics Express 23(11):A741-51 (2015) by Jiang et al., which isincorporated herein by reference in its entirety. The cyan and red peakswill account for a better visual acuity and perceived resolutioncompared to a spectrum having a peak around 555 nm. The perceivedcolourfulness or colour sensation is also increased: a viewer will beable to distinguish colours better and will perceive the colours as moreintense. At the same time, the S/P ratio is increased since the fractionof scotopic light (centred around 505 nm) is increased relative to thefraction of photopic light (centred around 555 nm).

Preferably, the blue peak of the third light source is limited inintensity, preferably less than 200% of the maximum of either of thepeaks of the first and second light sources. Too much blue light maydamage the retina of the eye and disturb the circadian rhythm byincreased melatonin suppression. Also, chromatic aberration of bluelight in the human eye may distort the visual acuity. Preferably, noyellow filter is needed to limit the strength of the blue emission peak.

The bandwidths of the peaks is narrow, which ensures a very limited orno overlap between the different peaks. The bandwidth is less than 15 nmfor at least one of the light sources. Preferably, the bandwidth is lessthan 14, 13, 12, 11, 10, 9, 8, 7, 6, or 5 nm, or between 3 and 15 nm,preferably between 3 and 10 nm.

Preferably, the peaks are spectrally symmetric, i.e. the intensityreduces with the same slope in both spectral directions from the peak.As a result, the dominant wavelength (which represents the colourobserved by the brain of a viewer) corresponds to the actual peakwavelengths.

The lighting device preferably comprises no light source with spectralcharacteristics other than the first, second and third light sources asrecited in claim 1. However, the lighting device may comprise aplurality of clusters comprising each of the three light sources. Thelighting device may comprise an array, which is a regular repetition ofthe three light sources or of the clusters. The array is preferablyrectangular but may also be a different array, such as a triangulararray. The smallest repeated grouping in the array is the cluster oflight sources; each cluster may comprise a first, a second and a thirdlight source, or at least one of each of the three light sources.Preferably, each cluster comprises only light sources of the first,second and third types and no light source having a different spectralcharacteristics. The screen may be provided with an array of at least 40clusters, preferably at least 80, more preferably at least 120.

In an embodiment, the screen further comprises spectral filterscorresponding to the peak wavelengths of the first, second and thirdlight sources, wherein each filter is configured to transmitsubstantially no light coming from light sources other than the lightsource to which it corresponds.

The light sources as described herein are beneficial in lighting deviceswith band pass filters, as compared to the case with conventional lightsources. They reduce the amount of light that would otherwise be blockedby the filters and also reduce leakage of light through the filters, forexample at the edges of the filters.

When the peaks are better segregated, it is much easier to filter outtwo of the three light sources, without contamination from the otherlight sources. The limited bandwidth thus further increases theperceived resolution and visual acuity. Furthermore, if filters areused, the narrow bandwidths allow for higher efficacy, since a smallerfraction of the light emitted by the light sources falls outside thetransmission range of (i.e. is blocked by) the filters.

In an embodiment, the screen is an LCD screen, such as a TFT-LCD screen,comprising a screen plane for displaying an image, the screen comprisinga pixel array with for each pixel three liquid crystals and three colourfilters, such as TFTs, corresponding to the first, second and thirdlight sources, and wherein the lighting device is configured to emitlight towards the pixel array, and wherein the liquid crystals arealigned with the colour filters so as to determine the transmission oflight from the lighting device towards the front plane; preferablywherein the screen further comprises a processing device for convertinga digital image to a collection of desired pixel colours, and acontroller for controlling the transmission of the liquid crystals,wherein the processing device is configured to communicate thecollection of desired pixel colours to the controller, and wherein thecontroller is configured to adapt the transmission of the liquidcrystals in each pixel to transmit light through a subset of the colourfilters, such that each pixel emits a desired colour according to thecollection of desired pixel colours.

The light sources may be LEDs and/or VCSELs which are preferablyinstalled in an array behind the screen or along the edges of thescreen.

The filters define coloured subpixels and are aligned to liquid crystalswhich are adapted to control the light intensity of the lighting deviceper coloured subpixel based on the image that is to be displayed. Thescreen may further comprise mutually perpendicular polarisers at bothsides of the liquid crystals.

The screen preferably includes a diffuser for scattering the lightemitted by the backlighting device, to increase the uniformness ofdirectional and spatial distribution of the light, wherein the diffuseris placed between the backlighting device and the liquid crystals. Thediffuser has the function to mix light from all light sources such thatno coloured spots can be seen on the screen.

Preferably, the resolution of the screen is at least 1080p (Full HD), atleast 2K or at least ultra-high definition (4K or 8K). It will beunderstood that the number of pixels may be higher than the number ofindividual clusters of light sources. In an embodiment, the number ofpixels may be at least 1000 times greater than the number of clusters,preferably at least 10000 times.

Materials used for the LEDs or VCSELs may for instance be GaAs orAlInGaP or AlGaAs for the first light source, InGaN or GaP for thesecond light source, and/or InGaN or GaN for the third light source, butalso other materials known in the art may be used. The person skilled inthe art will understand that the spectral distribution of the LEDs orVCSELs can be modified to the desired emission wavelengths. In order tolimit the bandwidth of the light sources, they may be equipped withnarrow band pass filters centred around the peak wavelength. Thesenarrow filters are distinguished from any filters associated with thecoloured pixels, since these narrow filters are associated with thelight sources itself (so the number is equal to the number of lightsources) and filter virtually all the light leaving the light source.The filters associated with the pixels are usually aligned with a liquidcrystal. Alternatively, the light sources may be equipped with quantumdots (QDs) or quantum wells (QWs) which also allow for narrow peaks.

Wherever the application mentions ‘screen’, alternative concepts such as‘display’, ‘interface’, ‘monitor’ and equivalents are intended to beincluded. The screens may be intended for televisions, mobile phones,notebooks, laptops, personal computers, tablets, vehicle interiors,watches, utility appliances, digital billboards, smartboards and otherequipment.

In an embodiment, at least one of the light sources is a vertical cavitysurface emitting laser light source (or, in short, VCSEL light source orVCSEL). A VCSEL is a very thin disk-shaped laser fabricated on a wafer,with light emitted by the top surface. Commercial electrically pumpedVCSELs can be made inexpensively and have a naturally round beam. VCSELscan be fabricated in arrays that emit very high powers.

In an embodiment, the third light source is a LED light source and thesecond light source is a VCSEL light source, and preferably the firstlight source is a VCSEL light source. In particular, only the secondlight source may be a VCSEL light source. Alternatively, the first lightsource and the second light source may be VCSEL light sources, while thethird light source is a LED light source. Alternatively, all three lightsources are VSCEL light sources.

Generally, the screen and backlighting device as described herein aresuitable for LiFi communication. LEDs or VCSELs allow for hightransmission rates and fast switching which is desired. In particular,VCSELs typically have a high cut off frequency, which facilitates highspeed when the lighting device is used for LiFi communication.

For the third light source (blue) a conventional LED may be used as blueLEDs have high efficiencies and are economically viable. In that case, ahybrid LED-VCSEL backlighting device may be formed which provides acombination of LEDs and VCSEL light sources. In combination, theyproduce the desired narrow emission peaks at high efficiencies.

For the second light source (which is cyan), VCSELs are very usefulsince LEDs that meet the wavelength and efficiency requirements are morechallenging to produce in an economical way. Possibly, the second lightsource may be a VCSEL of a shorter wavelength including a phosphorcoating or layer, such that the phosphor emits light that meets thespectral requirements (and the VCSEL light exciting the phosphor isblocked). It will be understood that apart from the fact that existingcyan LEDs have a relatively broad spectrum, the fact that this occupiesthe central frequency means that it potentially interferes with both ofthe other peaks from the first and third lighting sources. It alsocontributes to the spectral power at 555 nm, which is to be avoided.Choosing to replace the cyan LED with a narrow bandwidth VCSEL is thus avery cost effective improvement over existing lighting devices.

In an embodiment, the lighting device provides a spectral powerdistribution with a spectral power at 555 nm which is less than 50%,preferably less than 20%, more preferably less than 10% of the spectralpower at the peak wavelength of the first light source. Light emitted at555 nm does not effectively contribute to visual acuity so suppressingthe power at this wavelength increases overall visual efficiency.

In an embodiment, the lighting device is adapted to emit light in aspectral power distribution with an S/P ratio of between 2 and 5. TheseS/P ratios ensure an improved perceived intensity compared to (theconventional) lower S/P ratios.

In an embodiment, the CCT (correlated colour temperature) of thespectral power distribution is between 6000 and 7000 K, preferablyaround 6500 K. These CCTs correspond to natural daylight.

In an embodiment, the lighting device has a spectral power distributionwith a maximum spectral power, and the spectral power in the rangebetween 470 and 490 nm is less than 15% of the maximum spectral power,preferably with a minimum spectral power of less than 5% of the maximumspectral power; and/or the spectral power in the range between 550 and590 nm is less than 15% of the maximum spectral power, preferably with aminimum spectral power of less than 10% of the maximum spectral power.Such a spectral power distribution allows for a better perceived acuitybecause of better segregated peaks.

BRIEF DESCRIPTION OF THE DRAWINGS

The features and advantages of the invention will be further appreciatedupon reference to the following schematic drawings of a number ofexemplary embodiments, in which corresponding reference symbols indicatecorresponding parts.

FIG. 1A schematically shows a front view of a television screenaccording to an embodiment;

FIG. 1B schematically shows a cross-section of part of a televisionscreen according to an embodiment;

FIG. 2 schematically shows a spectral distribution of a lighting deviceaccording to the prior art;

FIG. 3 schematically shows a spectral distribution of a screen orlighting device according to an embodiment of the invention;

FIG. 4A schematically shows a spectral distribution of a screen orlighting device according to an embodiment of the invention;

FIG. 4B schematically shows a colour space or gamut corresponding to thespectral distribution in FIG. 4A;

FIG. 5 again schematically shows the spectral distribution of FIG. 4A,together with exemplary filter characteristics.

The figures are for illustrative purposes only, and do not serve as arestriction on the scope or the protection as laid down by the claims.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

FIG. 1A schematically shows a front view of an LCD television screen 1according to an embodiment, placed in a casing 2 in which the screen 1is mounted. The screen includes a pixel array 3 (only partly shown)built up of an array of pixels of which the colour can be setindividually. In order to show an image on the screen 1, a digital image(delivered by for instance a network server, a digital memory or acomputer) is converted by a processing device 4 to a collection ofcolours corresponding to the pixels in the pixel array 3. The processedimage is communicated to a controller 5 which controls the individualcolours of each pixel in the pixel array 3. Processing device 4 andcontroller 5 are preferably incorporated in the casing 2 of the screen 1itself, for instance at the back side of the screen. Alternatively, theyare separate from the screen 1 but are able to communicate with therelevant parts of the screen 1, in particular the pixel array 3 orspecifically the liquid crystals.

FIG. 1B schematically shows a cross-section of a part of the LCDtelevision screen. The figure shows a cross-section of a part of ascreen 1, excluding the casing in which the screen is mounted. At theback side, the screen 1 comprises a backlighting device 10 including anarray 11 of clusters 19 which each comprise a first light source 12, asecond light source 14 and a third light source 16. The light emittedfrom the lighting device passes through a diffuser 18 towards the liquidcrystals 30. The diffuser ensures that the light from the various lightsources is mixed, such that the colours are spatially and directionallyuniformly distributed over the screen. The liquid crystals 30 andfilters 40 are divided by subpixel 31 a, 31 b, 31 c, 32 a, 32 b, 32 c,41 a, 41 b, 41 c, 42 a, 42 b, 42 c so as to be able to vary the coloursby controlling the transmission through each filter 40 by means of itscorresponding liquid crystal 30. Three subpixels of the three differentcolours form a pixel 31, 41 in the pixel array. The liquid crystals 30and filters 40 are sandwiched in between mutually perpendicularpolarisers 20, 50. At the front side, a transparent cover layer 60limits the screen 1. The person skilled in the art will understand thatmany variations to the screen's components and layers can be conceivedwhich are compatible with the invention. In the illustrated case, thethree types of light sources 12, 14, 16 in the lighting device 10 aremounted in clusters 19 in an array 11 behind the liquid crystals 30.Alternatively, the clusters 19 may be placed only at the edges of thescreen 1, emitting into a waveguide which corresponds to the entirescreen surface. In another alternative embodiment, the light sources maynot be clustered. The working of exemplary backlit LCD screens has beenexplained in more detail for instance in U.S. Pat. No. 6,243,068, whichis incorporated herein by reference in its entirety.

FIG. 2 schematically shows a spectral distribution 200 of a lightingdevice according to the prior art. This spectrum is generated by awhite, phosphor conversion (PC) LED. The blue peak 215 corresponds tothe blue LED which excites the phosphor of which the phosphor emissionpeak 213 is centred around 555 nm. The blue peak 215 is weakened bymeans of a yellow filter, which leads to a loss of efficiency. It isclear from the spectrum that a large fraction of the light, inparticular most of the broad phosphor emission peak 213, is generatedfar from the central transmission wavelengths of the filters 241, 243,245 (indicated by the shaded rectangles). A large fraction of the lightis blocked by the filters and/or does not contribute to the perceivedresolution of a screen equipped with the lighting device.

FIG. 3 shows a spectral distribution 100 of a screen or lighting deviceaccording to an embodiment of the invention. The displayed spectraldistribution 100 corresponds to the intrinsic emission from the lightingdevice. The peaks 113, 115, 117 of the first, second and third lightsources each emit a narrow, substantially symmetric peak correspondingto the wavelengths of 625, 530 and 450 nm, respectively. In this casethe light sources are LEDs. The narrow peaks 113, 115, 117 ensure thatsubstantially all light can be used to form the coloured screen emissionand hardly any light is to be filtered out. In particular the peak 113of the first light source is narrow, with a FWHM of about 14 nm.

The illustrated spectral distribution 100 enables the screen to emitwhite light when desired, with a CCT (correlated colour temperature) ofabout 6500 K, corresponding to natural daylight.

FIG. 4A shows a spectral distribution 400 emitted by a screen orlighting device according to an alternative embodiment of the invention.The spectral distribution 400 comprises peaks 413, 415, 417,corresponding to first, second and third light sources, respectively. Inthis case, the light sources are VCSELs. The first light source emits apeak 413 around 625 nm, the second light source emits a peak 415 around525 nm, and the third light source emits a peak 417 around 450 nm. Thebandwidths (FWHM) of the three peaks are about 5 nm or less.

FIG. 4B shows a colour space or gamut 401 corresponding to the spectraldistribution in FIG. 4A. It can be appreciated that the colour space 401of the spectral distribution emitted by the claimed device (labelledSPD) is larger than the colour space 409 according to the HDTV (rec.709) standard or the colour space 408 according to the UHDTV (rec. 2020)standard, due to the wavelength of the second peak 415, which is shorterthan the corresponding peak in the standards (about 535 nm). Therefore,more different colours can be generated with a lighting device emittinglight with the SPD spectral distribution, compared to those emittinglight according to the standards.

FIG. 5 shows the spectral distribution 400 of FIG. 4A, with thecharacteristic transmission spectra 443, 445, 447 of the filterscorresponding to the respective peaks 413, 415, 417. The transmissionspectra 443, 445, 447 have very limited or substantially no overlap withthe peaks 413, 415, 417 of the light sources and are distant from the555 nm range.

The invention has been described by reference to certain embodimentsdiscussed above. It will be recognized that these embodiments aresusceptible to various modifications and alternative forms well known tothose of skill in the art.

Many modifications in addition to those described above may be made tothe structures and techniques described herein without departing fromthe spirit and scope of the invention. Accordingly, although specificembodiments have been described, these are examples only and are notlimiting upon the scope of the invention.

1. A backlighting device for a screen for a television, mobile phone orthe like, wherein the backlighting device comprises: a first lightsource adapted to emit light having a peak wavelength between 600 and630 nm; a second light source adapted to emit light having a peakwavelength between 510 and 530 nm; a third light source adapted to emitlight having a peak wavelength between 440 and 460 nm; wherein the lightemitted by one of the light sources has a bandwidth of less than 15 nm,preferably less than 10 nm, more preferably less than 5 nm.
 2. Thebacklighting device according to claim 1, wherein light emitted by thesecond light source has a bandwidth of less than 15 nm, preferably lessthan 10 nm, more preferably less than 5 nm.
 3. The backlighting deviceaccording to claim 1, wherein the light emitted by each of the lightsources has a bandwidth of less than 15 nm, preferably less than 10 nm,more preferably less than 5 nm.
 4. The backlighting device according toclaim 1, wherein the light emitted by the light sources forms peakswhich are spectrally symmetric.
 5. The backlighting device according toclaim 1, wherein the backlighting device provides a spectral powerdistribution with a spectral power at 555 nm which is less than 50%,preferably less than 20%, more preferably less than 10% of the spectralpower at the peak wavelength of the first light source.
 6. Thebacklighting device according to claim 1, wherein the backlightingdevice comprises a plurality of clusters, each of which comprises afirst light source, a second light source and a third light source. 7.The backlighting device according to claim 1, wherein the three lightsources are positioned in an array, preferably a rectangular array. 8.The backlighting device according to claim 1, wherein at least one ofthe light sources is a LED light source or wherein all light sources areLED light sources.
 9. The backlighting device according to claim 1,wherein at least one of the light sources is a VCSEL light source orwherein all of the light sources are VCSEL light sources.
 10. Thebacklighting device according to claim 1, wherein the third light sourceis a LED light source and the second light source is a VCSEL lightsource, and preferably wherein the first light source is a VCSEL lightsource.
 11. The backlighting device according to claim 1, wherein thebacklighting device is adapted to emit light in a spectral powerdistribution with an S/P ratio of between 2 and
 5. 12. The backlightingdevice according to claim 1, wherein the CCT of the spectral powerdistribution is between 6000 and 7000 K, preferably around 6500 K. 13.The backlighting device according to claim 1, wherein the backlightingdevice has a spectral power distribution with a maximum spectral powerand the spectral power in the range between 470 and 490 nm is less than15% of the maximum spectral power, preferably with a minimum spectralpower of less than 5% of the maximum spectral power.
 14. Thebacklighting device according to claim 1, wherein the backlightingdevice has a spectral power distribution with a maximum spectral powerand the spectral power in the range between 550 and 590 nm is less than15% of the maximum spectral power, preferably with a minimum spectralpower of less than 10% of the maximum spectral power.
 15. A screen for atelevision, mobile phone or the like, comprising the backlighting deviceaccording to claim
 1. 16. The screen according to claim 15, wherein thescreen is an LCD screen comprising a screen plane for displaying animage, the screen comprising a pixel array with for each pixel threeliquid crystals and three colour filters, such as TFTs, corresponding tothe first, second and third light sources, and wherein the backlightingdevice is configured to emit light towards the pixel array, and whereinthe liquid crystals are aligned with the colour filters so as todetermine the transmission of light from the backlighting device towardsthe front plane.
 17. The screen according to claim 16, wherein thescreen further comprises a processing device for converting a digitalimage to a collection of desired pixel colours, and a controller forcontrolling the transmission of the liquid crystals, wherein theprocessing device is configured to communicate the collection of desiredpixel colours to the controller, and wherein the controller isconfigured to adapt the transmission of the liquid crystals in eachpixel to transmit light through a subset of the colour filters, suchthat each pixel emits a desired colour according to the collection ofdesired pixel colours.
 18. The screen according to claim 16, whereineach filter is configured to transmit substantially no light coming fromlight sources other than the light source it corresponds to.
 19. Thescreen according to claim 16, wherein the screen further comprises adiffuser for scattering the light emitted by the backlighting device, toincrease the uniformness of directional and spatial distribution of thelight, wherein the diffuser is placed between the backlighting deviceand the liquid crystals.